1. Field of the Invention
The present invention relates to the etherification of C.sub.4 and C.sub.5 isoolefins, with an alcohol such as methanol, to produce the corresponding tertiary ether. More particularly the invention relates to a process wherein a catalytic distillation process is used in the process and wherein both isobutene and isoamylene are contained in the charge and wherein methanol is used to produce methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME).
2. Related Information
The increased governmental restriction on gasoline has strained the availability of feed stocks for etherification. The use of the isoamylenes for the preparation of octane improvers for gasoline has increased. It is highly desirable to be able to employ mixed isobutene/isoamylene streams. In a conventional separation of the reaction components from the etherification of a mixed C.sub.4 /C.sub.5 stream the separation of the unreacted C.sub.5 components from the MTBE is quite difficult because of their close boiling points. Even small amounts of MTBE in the C.sub.5 raffinate is detrimental to its use in a downstream gasoline alkylate plant.
U.S. Pat. No. 4,661,209 discloses the separation of MTBE from close boiling hydrocarbons by extractive distillation using a higher boiling oxygenated, nitrogenous and/or sulfur containing compound. The past necessity for such costly procedures has inhibited the actual feeding of C.sub.4 's and C.sub.5 's together to a reactor to co-produce MTBE and TAME. It has been found, however, that using a catalytic distillation system operated in a specific manner the separation can be achieved easily and concurrently in the reaction.
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873 all commonly assigned herewith. The isoolefins preferably react with the alcohol to form ethers. Briefly the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure, and also having a distillation zone containing inert distillation structure.
U.S. Pat. No. 5,248,837 discloses a method for controlling catalytic distillation etherifications wherein the methanol concentration below the catalyst bed is controlled to a point that maximizes ether production and prevents alcohol from leaving with the bottoms ether produce. The patent discloses only the reaction of one isoolefin and one alcohol at a time. In particular the invention is embodied by the production of TAME by the reaction of methanol and isoamylene.
Commonly assigned patent application Ser. No. 08/120,644 discloses a similar process to U.S. Pat. No. 5,248,837 wherein the alcohol content is controlled by total oxygen in the form of OH. This system is particularly useful when a mixture of alcohols is used.
Neither U.S. Pat. No. 5,248,837 nor application Ser. No. 08/120,644 address the problem of separation of MTBE from the unreacted overheads containing the unreacted C.sub.5 's.
It is an advantage of the present invention that MTBE product can be separated from the reactants when charging a mixed C.sub.4 /C.sub.5 stream to produce MTBE and TAME.